Amplifier linearisation

ABSTRACT

The present invention relates to a lineariser for use with an amplifier and to a method of linearising an amplifier. The lineariser comprises an input to receive an input signal and an output for outputting an adjusted signal. A gain variations adjustment means are provided for adjusting amplitude dependent gain variations of the signal. A phase variations adjustment means are provided for adjusting amplitude dependent phase variations of the signal. Said gain and phase variations adjustment means are adapted to be individually adjustable elements on the signal path between said input and output. A variable gain amplifier is provided on the signal path between the gain variations adjustment means and phase variations adjustment means. The adjusted signal is then input to said amplifier.

FIELD OF THE INVENTION

The present invention relates to linerisation of an amplifier, and inparticular, but not exclusively, to a lineariser for use with a poweramplifier for amplifying radio frequency (RF) signals. A lineariserimplemented in accordance with the invention may be used in elements ofa communication system.

BACKGROUND OF THE INVENTION

A communication system comprises signalling points or nodes, such asuser terminals, different exchanges, routers, switches, links, stationsand so on. The communication also comprises appropriate communicationmedia between the signalling points. Different signalling points may besituated within an element of the communication system, wherein thecommunication may occur with the element. The communication media maycomprise, for example, a wired interface, a radio frequency interface oran optical interface. The communication may be carried by analogue ordigital signals or a combination of these, such as digitally-modulatedanalogue signals.

Signal amplification is required in various communication applications.For example radio frequency signals transmitted between signallingpoints in a communication system employing radio transmission may needto be amplified during some stage of the transmission and/or reception.The signalling points may be, for example, a transmitting station and areceiving station or an intermediate node of the communication system.The amplification of the signals is required for example since theamplitude of a signal tends to be attenuated during transmission betweensignalling points, thereby decreasing the quality of the transmission.Also, noise becomes typically added to the signal from other sources aswell as from the transmitting and receiving and the possibleintermediate apparatus itself. A communication system is thus typicallyprovided with amplifying means to compensate for the attenuation.Amplification of the signal may also be used for increasing thesignal-to-noise ratio of the signal.

An input signal may be amplified by means of a power amplifier. Thepower amplifier may have at its input a modulated radio signal. Theinput radio signal may be a modulated digital signal. The poweramplifier, when operated in a non-linear region, typically close tosaturation produces intermodulation distortion products at its output aswell as the desired carrier signals. The intermodulation distortionproducts are typically produced on either side of the desired carriersignals. Intermodulation distortion products are typically caused by thepower amplifier not acting as a linear amplifier, which occurs when thepower amplifier is operating close to saturation. The intermodulationdistortion produces frequencies at multiples of the carrier frequency ofthe desired signals. These frequencies tend to be lower in signalstrength than the desired carriers. These intermodulation distortionproducts increase the spectral space occupied by the signal and aretherefore undesirable. One way to reduce the intermodulation distortionproducts is to operate the power amplifiers as linear amplifiers. Thepower amplifiers would be operated so that there is a substantiallylinear relationship between the input signal power and the output signalpower.

Amplifiers that are intended to cover a range of frequencies shouldprovide as linear performance as possible across the designatedfrequency band. However, an amplifier may introduce distortions. Thedistortion may be linear or non-linear. The linear distortions relate tobandwidth limitations. The non-linear distortions have two componentscalled AM-AM (amplitude modulation-amplitude modulation) or AM-PM(amplitude modulation-phase modulation) distortion. In the non-linearcase amplitude variations in the input signal may cause undesirableamplitude and/or phase variations in the output signal. In addition, thedistortions may cause mixing between the different frequency componentspresent in the signal. The term ‘AM-AM’ refers to amplitude dependentamplitude (gain) variations and the term ‘AM-PM’ refers to amplitudedependent phase variations.

A prior art solution for the linearity problem exploits the fact thatthe non-linearity increases with the output power level of theamplifier. Thus, if the input level is reduced, i.e. “backed-off”, theamplifier is arranged to operate only within its more linear region.However, this approach may not be desirable in all applications as itfails to utilise the full range of available output voltage-swing. Thebacking-off may have an disadvantageous effect on power efficiency ofthe amplifier.

The linearity of the amplifying function can also be improved byprovision of a linearisation function designed to reduce the distortion.Linearisers have been developed which enable operation of an amplifierwith reduced intermodulation distortion at a point on the gain/inputpower or phase/input power curves after which the amplifier does not actas a linear amplifier. Such a lineariser is placed in the signal pathbefore the power amplifier and may therefore precondition the inputsignal before passing it to the power amplifier. In other words the gainof the lineariser increases as the power of the input signal increasesand increases in such a way as to substantially oppose the typical poweramplifier power characteristics. This is called gain expansion. Inaddition, the linearizer may change the phase of the input signal as thepower of the input signal increases. The change in the phase may be suchthat it opposes the typical power amplifier phase characteristics. Thisis called phase expansion.

A lineariser should be capable of producing enough of gain and/or phaseexpansions to mitigate the non linear amplitude and phase problemsassociated with power amplifiers in a region that is close tosaturation. The effect of the lineariser is to increase the effectiverange over which the power amplifier is linear but allows the poweramplifier to operate in its more efficient non-linear range. However,the inventor has found that the prior art linearisers may not be capableof providing an optimised independent amplitude and phase tuning forlinearity and efficiency of the prior art lineariser arrangements. Inaddition, the prior art linearisers are of substantially big size,thereby making the integration thereof with the amplifier difficult inapplications requiring substantially small sized components, such asmobile phones.

SUMMARY OF THE INVENTION

Embodiments of the present invention aim to address one or several ofthe above problems.

According to one aspect of the present invention, there is provided alineariser for use with an amplifier comprising: an input to receive aninput signal; an output for outputting adjusted signal to saidamplifier; a signal path between the input and the output; a gainvariations adjustment means on the signal path for adjusting amplitudedependent gain variations of the signal; a phase variations adjustmentmeans on the signal path for adjusting amplitude dependent phasevariations of the signal, said gain and phase variations adjustmentmeans being individually adjustable elements; and a variable gainamplifier located on the signal path between the gain variationsadjustment means and phase variations adjustment means.

The arrangement is preferably such that the adjustment of the amplitudedependent gain variations does not produce any substantial amplitudedependent phase variations in the signal and/or vice versa.

The variable gain amplifier may be arranged for shifting amplitudedependent phase and gain variation curves relative to each other, andmore particularly, the variable gain amplifier may be provided forshifting the transfer characteristic curves of the amplitude dependentphase and gain variations relative to each other.

A second variable gain amplifier may be provided for adjusting the powerlevel of the signal to be input in said amplifier. A third variable gainamplifier may be provided for adjusting the power level of the inputsignal.

The gain variations adjustment means may comprise a field effecttransistor (FET). The phase variations adjustment means may be providedwith a non-linear resistor element.

The lineariser may be implemented as a microwave monolithic integratedcircuit.

The signal may be a radio frequency signal transmitted between a basestation and a mobile station of a cellular telecommunication system.

According to another aspect of the present invention there is provided amethod of linearising an amplifier, comprising: inputting a signal in alineariser; adjusting amplitude dependent gain variations of the signalindependently from amplitude dependent phase variations of the signal;adjusting amplitude dependent phase variations of the signalindependently from the adjustment of the amplitude dependent gainvariations of the signal, the amplitude dependent gain variations andthe amplitude dependent phase variations being adjusted by separateadjustment means; adjusting the power level of the signal by a variablegain amplifier located on the signal path between the gain variationsadjustment means and phase variations adjustment means; and outputtingthe signal from the lineariser to be input in said amplifier.

The embodiments of the invention may improve the linearity of a radiofrequency amplifier. The embodiments may also enable efficient use ofthe amplifying range of an amplifier. In addition, the embodiments mayenable an improved adjustment for the linearisation. The embodiments mayalso improve the possibilities to integrate a lineariser with anamplifier in applications requiring substantially small and/orintegrated components.

BRIEF DESCRIPTION OF DRAWINGS

For better understanding of the present invention, reference will now bemade by way of example to the accompanying drawings in which:

FIG. 1 shows one embodiment of the present invention;

FIG. 2 shows an amplitude dependent gain variations block for use in theembodiments of the invention;

FIG. 3 shows an amplitude dependent phase variations block for use inthe embodiments of the invention;

FIG. 4 illustrates gain and phase responses for the block shown in FIG.2;

FIG. 5 illustrates gain and phase responses for the block shown in FIG.3;

FIG. 6 illustrates the shifting between amplitude dependent gainvariations and amplitude dependent phase variations curves;

FIG. 7 is a flowchart illustrating the operation of one embodiment ofthe present invention; and

FIG. 8 shows a system wherein a lineariser in accordance with theinvention may be employed.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Reference is made to FIG. 1 which shows a block diagram of a lineariseri.e. predistorter 10 that has been implemented in accordance with anembodiment of the present invention. More particularly, FIG. 1 shows ananalogue predistorter configuration that is suitable for integrationwith an amplifier 5 so that they form a module. The predistorter isplaced before the amplifier on the signal path, i.e. such that thesignal output from the distorter element 10 is input to the amplifier 5.

The predistorter 10 is provided with two separate adjustment blocks 1and 2. By means of this the analogue predistorter 10 is adapted toenable independent or orthogonal adjustment of the AM-AM and AM-PMtransfer characteristics of the lineariser. By means of tuning the AM-AMand AM-PM characteristics of the lineariser it is possible to make thecascade of the lineariser and amplifier combination to behave in a morelinear manner.

More particularly, the predistorter 10 provides a possibility to tunethe AM-AM and AM-PM transfer characteristics independently relative toeach other. The independent adjustment can be accomplished such that thepower level point (so called knee point) where the AM-AM correctionstarts and the power level point where the AM-PM correction starts canbe shifted relative to each other. This relative shifting is illustratedin more detail by the diagrams of FIGS. 4 to 6.

As shown in more detail by FIG. 2, the AM-AM adjustment block 1comprises of a FET 11 (Field Effect Transistor). The arrangement is suchthat the AM-AM block 1 is adapted to produce only AM-AM gain expansionwhile the AM-PM transfer characteristics of the lineariser are keptconstant. This is illustrated by the two diagrams of FIG. 4.

The FET 11 is preferably biased in a pinch-off region. In other words,the FET 11 is preferably operated close to i.e. in the vicinity of thepinch-off region so that the gain expansion characteristics of the FET11 in this region can be utilised. The pinch-off region of the FET 11 isthe region where the FET 11 is just switched on. The pinch-off region istypically controlled by gate voltage.

The adjustment can be enabled by making it possible to tune the gate anddrain bias voltages 12, 13 of the FET 11. This does not influence theAM-PM characteristics of the block 1 and these remain substantially flat(see AM-PM/Power curve of FIG. 4). The tuning is accomplished such thatan AM-AM/Power curve as illustrated by FIG. 4 can be obtained. That is,the AM-AM block is tuned such that the AM-AM response of the cascadedlineariser and amplifier is made as flat as possible. In the usabletuning range of the gate and drain voltages the change of the shape ofthe AM-PM curve is negligible. The tuning range of the voltages ispreferably wide enough so that an improvement in the linearity can beobtained.

In the preferred embodiment the gate voltage is the main tuning voltage.The drain voltage may then be used for fine tuning. However, it shall beappreciated that the tuning is not restricted to use of only one voltagebut both of the drain and gate voltages can be used if this is deemednecessary.

The matching blocks 14, 15 are for matching the impedances of the FET 11to the impedances of other blocks connected to the AM-AM block 1.Inductor L, resistor R and capacitor C of FIG. 2 form a feedback network16 for the FET 11. The values of the components L, R and C arepreferably chosen such that a correct shape of the AM-AM can be obtainedso that the FET 11 can be tuned with the voltages, that as flat AM-PM aspossible is obtained, and that the matching of the FET 11 can be madeeasier.

The AM-PM adjustment block 2 is adapted to provide only AM-PMcharacteristics correction, that is a correction of the amplitudedependent phase variation. The correction is provided without affectingthe amplitude dependent gain i.e. the AM-AM transfer characteristics ofthe lineariser (see FIG. 5).

As shown in more detail by FIG. 3, the AM-PM block 2 consists of aforward biased diode 21 and a capacitor 22. Use of a non-linear resistorelement (represented by the diode 21) with capacitors enables provisionof an adjustment block that produces a substantially flat AM-AMresponse. The adjustment is adapted to be accomplished by tuning thebias voltage of the diode 21. More particularly, the diode 21 is shownto be supplied with bias voltage from a bias voltage source 23. Theadjustment is done by varying the bias voltage supplied by the source23. The bias voltage is used to tune the slope of the AM-PM transfercharacteristics of the block 2. This tuning is illustrated by the righthand side diagram of FIG. 5. A choke 24 may be provided for preventingthe RF signal from entering the bias network 23.

The predistorter 10 may also be provided with a first variable gainamplifier (VGA1) 3. The VGA1 can be used for setting the input powerlevel for the AM-PM block 2. The variable gain amplifier 3 may beprovided for shifting the AM-AM and AM-PM curves within the predistorter10 relative to each other. The diagram of FIG. 6 illustrates an exampleof the shifting of the AM-AM and AM-PM transfer characteristics relativeto each others by changing the gain of the first variable gain amplifier3.

The predistorter 10 may also be provided with a second variable gainamplifier (VGA2) 4. The second variable gain amplifier 4 can be used toset the power level of the input signal for the RF amplifier. Thevariable gain amplifier 4 may be provided for shifting the AM-AM andAM-PM curves of the predistorter 10 relative to the AM-AM and AM-PMcurves of the amplifier 5 so that an improved linearisation of theamplifier can be achieved. The variable gain amplifier 4 may enable amore optimal linearity improvement.

It shall be appreciated that the function of the variable gainamplifiers of FIG. 1 is not limited to amplification of signals. In someembodiment the VGAs may also be used for attenuation of the signals.

As shown by FIG. 7, in operation the tuning of the AM-AM transfercharacteristic may be accomplished by adjusting the gate and drainvoltages of the FET 11. The tuning the AM-PM transfer characteristic ispreferably accomplished first by adjusting the bias voltage supplied tothe diode 21. The input power level for the AM-PM block 2 may then beadjusted by changing the gain of a variable gain amplifier 3 in order toshift the AM-AM and AM-PM transfer characteristic curves and thus thepower points thereof relative to each others.

FIG. 1 shows also a third variable gain amplifier 8 (VGA3). Thisvariable gain amplifier may be used to adjust the power level of theinput signals. By the provision of the VGA3 it is possible to widen theoperational range of the lineariser 10, thereby enabling use thereof ina wide range of different applications. In other words, the variablegain amplifier 8 at the input of the lineariser 10 functions to set anappropriate correct input power level for the lineariser orpredistorter. By means of this the lineariser 10 can be fitted to avariety of systems which have different power levels.

The predistorter or lineariser 10 is believed to provide a compact,substantially cheap and easily adjustable lineariser for an RFamplifier. It is possible to avoid the shortcoming of the prior artlineariser in that the adjustments of the gain characteristicsinfluenced the phase characteristics and vice versa. The circuitryenables shifting of the AM-AM and AM-PM responses relative to eachother.

It shall be appreciated that the placing of the AM-AM adjustment block 1and the AM-PM adjustment block 2 is interchangeable, and should not haveany effect to the operation of the predistorter 10.

The predistorter may also be implemented as a Microwave MonolithicIntegrated circuit 9 (MMIC), as indicted in FIG. 1. This type ofimplementation may ease the integration of the predistorter into a PowerAmplifier (PA). MMICs as such are known and are used within many moderncircuits, for example in satellite and mobile telephony technologies.

Reference is now made to FIG. 8 illustrating a system in whichlinearisers in accordance with the invention may be employed. Theexemplifying system is a cellular mobile radio communication systemallowing a plurality of mobile stations MS1, MS2, MS3 to communicatewith a base (transceiver) station BTS in a common cell via respectivechannels CH1, CH2, CH3. The radio communication between a transmittingstation and a receiving station may be implemented in any appropriatemanner and may be based on any communication standard. Therefore theradio link as such will not be described in more detail herein. Examplesof cellular communication systems include, without being limited tothese, standards such as AMPS (American Mobile Phone System), DAMPS(Digital AMPS), GSM (Global System for Mobile communications), EDGE(enhanced data rate for GSM evolution), GPRS (General Packet RadioService), CDMA (Code Division Multiple Access), IS-95 or any of the3^(rd) generation (3G) communication systems, such as WCDMA (WidebandCDMA), UMTS (Universal Mobile Telecommunications System), or IMT-2000(International Mobile Telecommunications System 2000) and so on.

The mobile stations as well as the base station are provided withnecessary transceiver components (not shown in FIG. 8) so as to be ableto handle signals to be transmitted and received by respective antennae.These components are known to a skilled person and do not as such form apart of the invention, and are thus not described in more detail. It issufficient to note that the components of a communication systemtypically include one or several power amplifiers.

The power amplifiers of the base station BTS or the mobile stations ofFIG. 8 may be provided with linearisers as discussed above. However, itshould be appreciated that the embodiments of the present invention areapplicable to any other suitable type of equipment employing amplifiers.

It is also noted herein that while the above describes exemplifyingembodiments of the invention, there are several variations andmodifications which may be made to the disclosed solution withoutdeparting from the scope of the present invention as defined in theappended claims.

1. A lineariser for use with an amplifier comprising: an input toreceive an input signal; an output for outputting adjusted signal tosaid amplifier; a signal path between the input and the output; a gainvariations adjustment means on the signal path for adjusting amplitudedependent gain variations of the signal; a phase variations adjustmentmeans on the signal path for adjusting amplitude dependent phasevariations of the signal, said gain and phase variations adjustmentmeans being individually adjustable elements; and a variable gainamplifier located on the signal path between the gain variationsadjustment means and phase variations adjustment means, wherein saidvariable gain amplifier is for shifting the transfer characteristiccurves of the amplitude dependent phase and gain variations relative toeach other.
 2. A lineariser as claimed in claim 1, the arrangement beingsuch that the adjustment of the amplitude dependent gain variations doesnot produce any substantial amplitude dependent phase variations in thesignal.
 3. A lineariser as claimed in claim 1, the arrangement beingsuch that the adjustment of the amplitude dependent phase variationsdoes not produce any substantial amplitude dependent gain variations inthe signal.
 4. A lineariser as claimed in claim 1, wherein the turningpoint of the amplitude dependent phase variations curve is adapted to beshifted by the variable gain amplifier.
 5. A lineariser for use with anamplifier comprising: an input to receive an input signal; an output foroutputting adjusted signal to said amplifier; a signal path between theinput and the output; a gain variations adjustment means on the signalpath for adjusting amplitude dependent gain variations of the signal; aphase variations adjustment means on the signal path for adjustingamplitude dependent phase variations of the signal, said gain and phasevariations adjustment means being individually adjustable elements; anda variable gain amplifier located on the signal path between the gainvariations adjustment means and phase variations adjustment means,wherein a second variable gain amplifier is provided for adjusting thepower level of the signal to be input in said amplifier.
 6. A lineariserfor use with an amplifier comprising: an input to receive an inputsignal; an output for outputting adjusted signal to said amplifier; asignal path between the input and the output; a gain variationsadjustment means on the signal path for adjusting amplitude dependentgain variations of the signal; a phase variations adjustment means onthe signal path for adjusting amplitude dependent phase variations ofthe signal, said gain and phase variations adjustment means beingindividually adjustable elements; and a variable gain amplifier locatedon the signal path between the gain variations adjustment means andphase variations adjustment means, wherein a third variable gainamplifier is provided for adjusting the power level of the input signal.7. A lineariser as claimed in claim 1, wherein the gain variationsadjustment means comprises a field effect transistor (FET).
 8. Alineariser as claimed in claim 7, wherein the amplitude dependent gaincharacteristics of the signal are adjustable by adjusting the gatevoltage of the field effect transistor.
 9. A lineariser for use with anamplifier comprising: an input to receive an input signal; an output foroutputting adjusted signal to said amplifier; a signal path between theinput and the output; a gain variations adjustment means on the signalpath for adjusting amplitude dependent gain variations of the signal; aphase variations adjustment means on the signal path for adjustingamplitude dependent phase variations of the signal, said gain and phasevariations adjustment means being individually adjustable elements; anda variable gain amplifier located on the signal path between the gainvariations adjustment means and phase variations adjustment means,wherein the gain variations adjustment means comprises a field effecttransistor (FET), and wherein the amplitude dependent gaincharacteristics of the signal are adjustable by adjusting the drainvoltage of the field effect transistor.
 10. A lineariser as claimed inclaim 7, wherein biasing means are provided to provide the field effecttransistor with a biasing voltage.
 11. A lineariser as claimed in ofclaim 10, wherein the field effect transistor is biased in a pinch-offregion.
 12. A lineariser as claimed in claim 10, wherein amplitudedependent gain variations of the signal are adapted to be adjusted bymeans of adjusting biasing voltage of the field effect transistor.
 13. Alineariser for use with an amplifier comprising: an input to receive aninput signal; an output for outputting adjusted signal to saidamplifier; a signal path between the input and the output; a gainvariations adjustment means on the signal path for adjusting amplitudedependent gain variations of the signal; a phase variations adjustmentmeans on the signal path for adjusting amplitude dependent phasevariations of the signal, said gain and phase variations adjustmentmeans being individually adjustable elements; and a variable gainamplifier located on the signal path between the gain variationsadjustment means and phase variations adjustment means, wherein the gainvariations adjustment means comprises a field effect transistor (FET).wherein biasing means are provided to provide the field effecttransistor with a biasing voltage, and wherein the biasing meanscomprise a first biasing means for biasing the gate voltage and a secondbiasing means for biasing the drain voltage of the field effecttransistor.
 14. A lineariser as claimed in claim 1, wherein the phasevariations adjustment means is provided with a non-linear resistorelement.
 15. A lineariser as claimed in claim 14, wherein the phasevariations adjustment means comprises a biasing voltage source forprovision of a biasing voltage for the non-linear resistor element. 16.A lineariser as claimed in claim 14, wherein the non-linear resistorelement comprises a forward biased diode.
 17. A lineariser as claimed inclaim 15, wherein the amplitude dependent phase variations of the signalare adapted to be adjusted by adjusting the biasing voltage of thenon-linear resistor element.
 18. A lineariser as claimed in claim 1,wherein the phase variations adjustment means is provided with capacitormeans.
 19. A lineariser as claimed in claim 1 being implemented as amicrowave monolithic integrated circuit.
 20. A lineariser as claimed inclaim 1, wherein said input signal is a radio frequency signal.
 21. Alineariser as claimed in claim 20, wherein said radio frequency signalis a signal to be transmitted between a base station and a mobilestation of a cellular telecommunication system.
 22. A lineariser asclaimed in claim 1, wherein said at least variable gain amplifier isadapted to amplify signals.
 23. A lineariser as claimed in claim 1,wherein said at least variable gain amplifier is adapted to attenuatesignals.
 24. A lineariser as claimed in claim 1 in combination with anamplifier.
 25. Communications apparatus comprising a lineariser asclaimed in claim
 1. 26. A communications apparatus as claimed in claim25, wherein said communications apparatus is used for communication in acellular communications system.
 27. A communications apparatus asclaimed in claim 26, wherein said apparatus is a mobile station.
 28. Acommunications apparatus as claimed in claim 26, wherein said apparatusis a base station.
 29. An integrated circuit comprising a lineariser asclaimed in claim
 1. 30. An integrated circuit as claimed in claim 29,wherein said integrated circuit is a microwave monolithic integratedcircuit.
 31. A method of linearising an amplifier, comprising: inputtinga signal in a lineariser; adjusting amplitude dependent gain variationsof the signal independently from amplitude dependent phase variations ofthe signal; adjusting amplitude dependent phase variations of the signalindependently from the adjustment of the amplitude dependent gainvariations of the signal, the amplitude dependent gain variations andthe amplitude dependent phase variations being adjusted by separateadjustment means; adjusting the power level of the signal by a variablegain amplifier located on the signal path between the gain variationsadjustment means and phase variations adjustment means; and outputtingthe signal from the lineariser to be input in said amplifier, furthercomprising the step of adjusting the relative position between kneepoint of the amplitude dependent phase variations and the knee point ofthe amplitude dependent gain variations.
 32. A method as claimed inclaim 31, wherein the knee point of the amplitude dependent phasevariations is adjusted.
 33. A method as claimed in claim 31, wherein therelative positioning of the knee points is adjusted by said adjustmentof the power level of the signal.
 34. A method as claimed in claim 31,comprising adjustment of a gate voltage of a transistor provided for theadjustment of the amplitude dependent gain variations.
 35. A methodlinearising an amplifier, comprising: inputting a signal in alineariser; adjusting amplitude dependent gain variations of the signalindependently from amplitude dependent phase variations of the signal;adjusting amplitude dependent phase variations of the signalindependently from the adjustment of the amplitude dependent gainvariations of the signal, the amplitude dependent gain variations andthe amplitude dependent phase variations being adjusted by separateadjustment means; adjusting the power level of the signal by a variablegain amplifier located on the signal path between the gain variationsadjustment means and phase variations adjustment means; and outputtingthe signal from the lineariser to be input in said amplifier, andfurther comprising adjustment of a drain voltage of a transistorprovided for the adjustment of the amplitude dependent gain variations.36. A method as claimed in claim 31, comprising adjustment of a biasingvoltage supplied to a non-linear resistor element provided for theadjustment of the amplitude dependent phase variations.